1. Field of the Invention
The invention relates to temperature sensors. More particularly, the invention relates to integrated circuit (IC) temperature sensors having linear response, adjustable gain and adjustable offset.
2. Description of the Related Art
Integrated circuit (IC) temperature sensors are used to determine the die or substrate temperature of an integrated circuit in which the temperature sensors are embedded. Since these temperature sensors are on the same die as the active circuitry whose temperature is to be measured, the temperature sensors can determine the junction temperature (the primary temperature of interest) of the active circuitry more accurately than, e.g., measuring the external IC package temperature. In general, it is known that the base-emitter voltage Vbe of a forward-biased transistor is a linear function of absolute temperature (T) in degrees Kelvin (° K), and is useful as the basis for a stable and relatively linear temperature sensor.
One such type of conventional temperature sensor typically involves a bandgap circuit that generates a current proportional to absolute temperature (IPTAT), which, after being scaled by a current mirror or other suitable arrangement, is sourced through a temperature sensor resistor RTS to provide the output voltage VOUT of the temperature sensor. Since the temperature sensor resistor RTS is the same type of resistor as the resistor (RPTAT) used in the bandgap circuit to generate IPTAT, the output of the temperature sensor circuit is linearly proportional to temperature. Also, the gain (in volts/degrees Kelvin) of the temperature sensor, which is a function of the ratio of the temperature sensor resistor RTS to the bandgap circuit resistor RPTAT, is adjustable. However, in this type of conventional temperature sensor, there is no voltage offset, i.e., the output voltage at 0° K (−273° Celsius) is 0 volts.
Some other types of conventional temperature sensors offer both adjustable gain and adjustable offset. For example, see the temperature sensor disclosed in Pease, “A New Fahrenheit Temperature Sensor,” IEEE Journal of Solid-State Circuits, Vol. SC-19, No. 6, December 1984, pages 971-977. However, the temperature sensor in Pease requires that the gain be calibrated by trimming the offset error at room temperature. That is, one or more resistors, e.g., the resistor R4 and a resistor in the IPTAT current source circuit, must be trimmed to calibrate the gain.
Trimming, e.g., resistor trimming, is a conventional technique for calibrating sensor circuit performance. It involves, e.g., including a network of fusable links or buried fuses in the circuit to modify resistor values by blowing one or more of the fuses. Although trimming helps to compensate for component tolerances, manufacturing variations, and the effects of temperature and aging, trimming is relatively costly, time consuming in terms of extra test time for calibration, and requires additional process technology, e.g., process technology that supports trim links, fuses and other forms of one-time programmable devices.
Another conventional temperature sensor is disclosed by Audy in U.S. Pat. No. 5,529,354. The temperature sensor in Audy offers a less complex circuit design than the temperature sensor disclosed in Pease, and provides a programmable voltage offset for the temperature sensor. The programmable offset is provided by adding an offset resistor to a conventional band gap temperature cell and by generating the sensor output voltage at a different point in the circuit. However, to program the desired offset, Audy too requires trimming the offset resistor. Also, to program the gain of the temperature sensor, Audy requires trimming another resistor in the bandgap cell. As discussed hereinabove, resistor trimming is not an efficient calibration or offset adjustment technique.
Accordingly, it would be desirable to have an IC temperature sensor that generates an output voltage that is linearly proportional to the IC die temperature and also allows for both the gain and the voltage offset to be adjusted independently by IC parameters, without trimming.